1. Field of the Invention
The present invention relates to an encoding circuit with a function of zero continuous-suppression (below, a zero continuous-suppression encoding circuit). Particularly, it relates to a zero continuous-suppression encoding circuit for converting a NRZ (non-return-to-zero) signal to two serial signals corresponding to each pole of a bipolar pulse signal which indicates a desirable zero continuous-suppression code.
2. Description of the Related Art
In a transmission system using a baseband transmission, for example, a SONET (Synchronous Optical Network) or SDH (Synchronous Digital Hierarchy), it is necessary to suppress direct current (DC) components contained in the signals transmitted to the transmission line. When the DC components can be suppressed preferably, it is possible to easily (extract bit synchronous information contained in she signals, and to compress the transmission bandwidth of the signals. As a result, it is possible to quickly detect various faults caused by the transmission line or transmission apparatus at high precision. Accordingly, the above performances are always required in the field of transmission systems.
In order to realize suppression of DC components, a zero continuous-suppression code is applied to the transmission system. The zero continuous-suppression code is used for converting a zero pattern consisting of continuous several bits to particular code pattern, for example, B3ZS (Bipolar With 3 Zeros Substitution) code or HDB3 (High Density Bipolar 3) code.
There are, however, various problems in the zero continuous-suppression encoding circuit when adding this circuit to an LSI circuit as explained in detail below.